1. Field of the Invention
This invention relates to an apparatus suitable for measuring the thickness of sheet-like products such as magnetic tapes, particularly the thickness of the coating thickness on them.
2. Description of the Prior Art
An apparatus utilizing electric vortex current as shown in FIG. 1 is already known for measuring the thickness of sheet-like products such as audio tapes, video tapes, etc., particularly the thickness of magnetic materials as a coating coated on the tape-like supports thereof made of polyester and other plastics.
In FIG. 1, reference numeral 1 designates a magnetic tape, in which tape-like support layer 1a made of transparent polyester and others is provided with a coating layer 1b of magnetic materials coated on a predetermined surface thereof, while 14 is a detector comprising an iron core 14a with one open end, which is wound around by exciting coil 14b connected with an oscillator 15.
Now the functions of the apparatus will be described. Referring to FIG. 1, when the oscillator 15 is excited by the exciting coil 14b, magnetic field due to the exciting coil 14b is generated in the open end of the iron core 14a. Therefore, the exciting impedance fluctuates when a coating layer 1b, namely, magnetic material takes an adjacent position as shown, since fluctuation of the exciting impedance depends upon characteristics, sizes (width, thickness), position, etc., of the magnetic material, the coating thickness of the magnetic material, i.e., the coating layer 1b can be measured by detecting the impedance fluctuation under the predetermined condition of the characteristics of the magnetic material, as well as the width and the position thereof.
It is difficult for the conventional apparatus with the construction mentioned above to measure reliably the thickness of the coating layer if the distance between the coating layer and the detector can't be kept constant and narrow, therefore, the conventional apparatus has a disadvantage in that it is practically difficult to be installed in production lines.
On the other hand, an optical micrometer is also known, wherein light beams such as laser beams are utilized with the body thereof positioned relatively apart from the object whose thickness is to be measured.
By using such apparatus, it is possible to measure the thickness after coating the magnetic material and then determine the coating thickness when the tape-like support layer thereof is controlled at a constant thickness.
Examples of such measuring apparatus are disclosed in U.S. Pat. No. 4,082,463 as well as Japanese Patent Laid Open Nos. 57-161,608 (1982) and 57-104,807 (1982), wherein the construction shown in FIG. 2 is adopted principally.
Referring to FIG. 2, reference numeral 6 is a scanner such as polygonal mirror and a light beam from a light source 4 is scanned upward and downward onto a first lens 52, which irradiates a parallel beam onto a second lens 55, while the parallel beam reaches the second lens 55, through a gate 53 comprising iris or restrictors 53a, 53b and collected by a receiving surface of a photoelectric converter 7. Therefore, when the light beam is scanned at a constant speed from the scanner 6 onto the first lens 52, the photoelectric converter 7 receives the light beam travelling up and down in the span of the gate 53.
An object 54 to be measured is positioned between the first and second lenses 52, 55, therefore, the photoelectric converter 7 doesn't receive any light beam which remains in a travelling route covered by the vertical dimension of the object 54 to be measured. The vertical direction (a distance between the iris or the restrictor 53a and another restrictor 53b) thereof being known, it is possible to measure the vertical (on the FIG. 2) dimension of the object 54 if the time interval from initial incidence of the light beam onto the photoelectric converter 7 to the interruption thereof, the time interval of the interruption and the time interval from reincidence of the light beam to the reinterruption thereof can be detected respectively.
Moreover in such apparatus, as shown in FIG. 3, the object 54 to be measured is substituted with a shaft, which supports a magnetic tape by being partly rounded with ascending thereonto and then descending therefrom, so that a clearance between the upper iris or the restrictor 53a and the top surface of the magnetic tape 1 supported by the shaft 2 can be measured. However, such apparatus has the following disadvantage for some configuration and reflectance of the object to be measured (the magnetic tape 1 in this case), for instance, if the magnetic tape 1, the object to be measured is cylindrical with high surface reflectance and the light beam is projected upon a part (as identified by "d") near to the top cylinder-shaped part of the magnetic tape, the reflection 59 thereof is projected upon the light receiver 7.
Therefore, in spite of the actual presence of the magnetic tape 1 as the object to be measured, the light receiver 7 gives the detection result as if the magnetic tape 1 is not present. Moreover, the aforementioned influences of the reflected light beam depend upon the configuration of the object to be measured (the curvature difference of the magnetic tape 1 due to the diameter difference of the shaft 2), the surface reflectance thereof, etc., resulting in difficult error compensation.